1. Field
The present application generally relates to computer-generated animation, and, more particularly, to using variable frame rates in rendering and projecting a computer-generated animation sequence.
2. Related Art
When a succession of still images is viewed, the separate frames appear continuous and the objects in the frames appear to move in smooth motion. It has been found that a frame rate of 10 frames per second is sufficient to give the illusion of motion. However, the fewer frames per second, the less smooth the motion appears. For example, with a frame rate of 16 frames per second, the images may flicker or strobe. Flickering and strobing effects generally occur when the eye detects the individual frames of the picture, because the time between the frames is too long or the degree of motion with consecutive frames is too great.
Traditional films have a frame rate of 24 frames per second, but film projectors normally shutter at twice this rate (i.e., each frame is flashed twice), thus achieving a 48 Hertz refresh rate. To some extent, the higher refresh rate reduces the flickering and strobing effects without requiring more film, but audiences may still see a considerable amount of flicker in some cases.
One solution to the flickering or strobing problem is to increase the frame rate, e.g., to 48 frames per second or 60 frames per second. This improves the motion picture quality, but requires more film and is thus more costly. For example, U.S. Pat. Nos. 4,560,260 and 4,477,160, entitled “Motion Picture System,” disclose a process for producing and projecting a motion picture, where the film frames are photographed and projected at a predetermined constant rate of more than 50 frames per second, such as 60 frames per second.
In another example, a commercial product known as the Maxivision system runs either at 24 frames per second or at 48 frames per second, which reduces strobing effects. The system uses normal 35 mm motion picture film, but captures images with a non-standard number of perforations of film per frame. While the amount of additional film needed to achieve the higher frame rate is reduced, it requires using fewer perforations of film per frame than the standard. See, U.S. Pat. No. 5,745,213, entitled “Method of making enhanced resolution motion picture release-print film.”
Computer-generated animation can have similar flickering and strobing problems, especially during high-action or high-motion scenes. The speed at which objects appear to move in an animation is a function of the number of drawings used to obtain a movement and the distance between the object's positions in successive frames. For example, when animating a bouncing ball, the further the ball has moved in each adjacent frame, the faster the ball will appear to travel across the screen. If there is too much distance between the ball's positions in successive frames, the ball will appear to jump from one spot on the screen to another, rather than move smoothly.
In recent years, digital projection has paved the way for a new generation of three-dimensional (3D) cinema. However, 3D filmmakers also face problems with flickering and strobing effects, especially in high-action or high-motion scenes. A stereoscopic imaging system creates the illusion of depth in an image by presenting a slightly different image to each eye. The two slightly different images are called a stereo pair. In addition, only one eye is presented with an image at a time. Unfortunately, the alternating presentation of left and right eye imagery and the frequency at which the images are updated make the flickering and strobing effects in 3D cinema even more pronounced.
To alleviate part of the problem, the D-Cinema consortium, which focuses on digital cinema equipment certification, requires D-Cinema-compliant 3D projectors to implement a technique known as “triple flashing” of the stereo pair of images as a solution to perceptible flicker issues inherent in lower frequency refresh rates. With single-flashing, the left image is projected onto the screen for 1/48 second, and then the right image is projected onto the screen for another 1/48 second. Single-flashing presents to each eye a flicker rate, which is clearly perceptible and causes discomfort in the viewer. With double-flashing, the left image and the right image are alternately projected onto the screen, each staying on the screen for 1/96 second, and then the stereo pair is flashed the same way a second time. Double-flashing is an improvement, but still yields perceptible flicker for some viewers under some circumstances. With triple-flashing, the left image and the right image are alternately projected onto the screen, each staying on the screen for 1/144 second, and then the stereo pair is flashed the same way a second and a third time. Under normal circumstances, triple-flashing provides a flicker frequency, which is imperceptible to a viewer. Unfortunately there are still undesirable consequences, which result from the alternating sequential presentation of left and right images multiple times.
Fast lateral movements of the subjects or the camera may create a marked and uncomfortable strobing or stuttering effect. Lateral image displacement resulting from these fast lateral subject or camera movements may become indiscernible from lateral image displacement due to the stereo offset. As a result, the audience may experience spatial confusion and/or discomfort. The root cause of this undesirable situation lies in the repeated presentation of each image (three times) and the related lack of updated or fresh imagery.